To my relief, I am not causing Izzy's tension, but I am not necessarily helping it either. Sean suggested some things that I can do to help Izzy build confidence in me as his rider.
1) Insist that he let go through his poll by using lateral flexion with an inside leg. Izzy does not get to deviate from that position. As he releases the tension, I can straighten him for a few strides and let him move into the contact, but then it's back to lateral flexion. As Izzy finds relief in this position, he will seek it more willingly.
2) I need to ride Izzy with contact all the time. He doesn't get a loose, free rein as this is when he looks for things at which to spook. Instead, I can use walk "breaks" to reinforce the idea of letting go through the poll.
3) When Izzy spooks, I am to ignore it completely and ride as though nothing happened.
4) For now, I am to stick with the larger movements: 20-meter circles, serpentines, and figures of eight. These will help keep Izzy balanced as he builds trust in me as his rider.